1. Field of the Invention
The present invention relates to a cylindrical dynamic damper intended for mounting on a rotating shaft, such as an automobile drive shaft or a propeller shaft for example, in order to suppress detrimental vibration produced by the rotating shaft.
2. Description of the Related Art
The rotating shafts, such as an automobile drive shaft or a propeller shaft, may experience undesired detrimental vibration such as bending vibration or twisting vibration due to rotational unbalance produced in association with rotation thereof. In order to suppress such detrimental vibration, dynamic dampers of various kinds are employed. A dynamic damper achieves its function by means of matching its resonance frequency (natural frequency) to the predominant frequency of the detrimental vibration produced by the rotating shaft. With this arrangement, the vibrational energy of the rotating shaft is absorbed by being converted to vibrational energy of the dynamic damper.
JP-A-2004-92674, JP-A-9-89047 and JP-A-2002-98193 (hereinafter referred to as “Citations 1, 2 and 3”, respectively) teach examples of such dynamic dampers, each comprising: a cylindrical mass member disposed spaced a distance apart from an outside periphery of a rotating shaft in a coaxially fashion; and a pair of rubber elastic support members. The pair of rubber elastic support members includes a pair of ring-shaped affixing members situated at axial ends of the mass member and mounted on the outside peripheral face of the rotating shaft, and a pair of elastic support portions of tapered cylindrical shape connecting respectively to the affixing members and to the axial ends of the mass member to thereby elastically support the mass member on the rotating shaft.
Citations 1 and 2 teach that the rubber elastic support members are imparted with variable wall thickness and axial length to form high spring parts whose spring constant in the axial direction is higher than a certain value and low spring parts whose spring constant is lower than the aforementioned certain value. These high spring parts and low spring parts are situated in alternating fashion in the circumferential direction. Citation 1 also teaches that at the axial ends of the mass member, a recessed end face recessed axially inward and connected to a first end of a low spring part, and a convex end face located axially outward from the recessed end face and connected to a first end of a high spring part are formed in alternating fashion in the circumferential direction. With this arrangement, the dynamic dampers taught in Citations 1 and 2 can set a plurality of resonance frequencies thereof over a wide range, bridging a single target resonance frequency of the rotating shaft. This makes it possible to hold to a minimum the drop in vibration damping action produced by variability in the resonance frequency of the rotating shaft and of the cylindrical dynamic damper. The resonance frequency of a cylindrical dynamic damper is basically determined by the mass of the mass member and the spring constant of the rubber elastic support members in the axis-perpendicular direction.
Citation 3 discloses an arrangement wherein, at the inside peripheral corners at the axial ends of the mass member, a slanted face that extends in a chamfered profile along the axial end face and the inside peripheral face of the mass member is formed in a continuous tapered cylindrical configuration about the circumference, with one axial end of the rubber elastic support member affixed to the slanted face. This arrangement makes it possible for the rubber elastic support member to be shifted and positioned axially inward, thus assuring adequate effective length of the rubber elastic support member, while minimizing the distance over which the mass member projects in the axial direction from the axial end face in the rubber elastic support member. Thus, it is possible to achieve both sufficient mass on the part of the mass member, and compact size.
It is known that rotating shafts, such as the drive shaft installed in an automobile, can experience detrimental vibration due to vibration transmitted via the wheels. Thus, when tuning the resonance frequency of a dynamic damper installed on the rotating shaft, the resonance frequency (natural frequency) of the wheels must be taken into consideration. However, wheels commonly used can be generally divided into steel ones and aluminum ones, and the resonance frequency of steel wheels is appreciably different from the resonance frequency of aluminum wheels, with the resonance frequency of aluminum wheels being about 1.5 time or more that of steel wheels.
Thus, in the cylindrical dynamic dampers of the type taught in Citations 1 and 2, the high spring portions and the low spring portions of the rubber elastic support members are designed to support the mass member. Accordingly, if it attempted to tune the damper to the two resonance frequencies of steel wheels and aluminum wheels, even if the thickness and free length of these portions are adjusted to the maximum possible extent, the ability to effect tuning such that the resonance frequency on the high frequency end is about 1.5 times the resonance frequency on the low frequency end is limited.
In a dynamic damper having a construction in which the axial ends of the mass member are elastically supported by a pair of rubber elastic support members, stress tends to become concentrated in areas where the rubber elastic support members are connected to the mass member. Further, the additional stress concentration may be produced due to the presence of edge portions (inside peripheral corners) of the mass member in these connection areas, posing the risk of difficulty in ensuring adequate durability of the rubber elastic support members. With respect to this point, Citation 1 shows that a recessed end face and a convex end face disposed in alternating fashion in the circumferential direction are formed in a stepped configuration in the axial direction on the two axial ends of the mass member, thereby creating a large spring ratio between the high spring parts and the low spring parts. With this arrangement, the step between the recessed end face and the convex end face tends to become rather large, which is disadvantageous in terms of assuring adequate durability.